Many passive safety belt systems have been devised heretofore for ensuring the safety of the vehicle occupants. For example, in the passive belt system of FIG. 3 of the drawings, a shoulder belt 4 leads from a belt retractor 5 on the inboard side of the seat 3 of a vehicle 1 to an emergency release buckle 6 at the free end 7 of the belt. The buckle is connected to a tongue carried by a belt guide member 9 that moves along a guide rail 8 installed at the edge of the vehicle roof. A knee bar protects the occupant's lower body. As described below, the shoulder belt is moved automatically between the occupant restraint and release positions in response to opening and closing the vehicle door 2.
The inventor of the present invention previously proposed a passive safety belt system of such a type in Japanese Utility Model Publications Nos. 108668/1986 and 193849/1986. The device disclosed in the former publication, which is an improved type of the latter, is shown in FIGS. 1 and 2. In that drive device, the belt guide 9 (FIG. 3) is attached to the free end of a flexible drive tape 14. The other end of the drive tape 14 and one end of a driving band 16 are attached to a first drum 15 such that they may be wound and unwound in an overlapped state, each turn of the band 16 overlying the corresponding turn of the tape 14. A second drum 18 is located adjacent the path 17 along which the tape 14 leads from the first drum 15. The driving band 16 separates from the tape 14 and has its other end affixed to the second drum. An interlocking mechanism 19, such as gears, couples the shafts of the first and second drums 15 and 18 so that they rotate conjointly, and a driving mechanism 20 is connected to at least one of the shafts 21 and 22 of the first and second drums 15 and 18 and rotates the drum shafts 2 and 22 selectively in either direction. The shaft 22 of the second drum 18 is fixed to its gear and is thus driven directly at all times when the device is operating. The first drum 15 is rotatable on its shaft 21 but is coupled by springs 23 to the shaft 21 that bias it in the winding direction of the drum. The springs 23 allow the first drum to rotate at a slightly different speed than that of the shaft, thereby accommodating the difference between the rate of change in the thickness of the tape and band on the drum roll 15 and the rate of change in thickness of the band on the drum 18 as the tape and band wind and unwind from the respective drums. The springs 23 enable the tape and band to wind tightly onto the drum 15 by maintaining a bias in the winding direction.
In particular, even though the first and second drums 15 and 18 have a constant relative rotation ratio due to the interlocking mechanism, their actual diameters when winding or unwinding the flexible drive tape 14 and the driving band 16 increase or decrease according to the thickness of the wound tape or band. The change in the diameter of the first drum 15 as the tape and band wind and unwind in the overlapped state is larger than that of the second drum 18 as the band 16 alone winds and unwinds, and there is a chance of slack or looseness developing in the band 16 when it is wound onto the second drum 18. Such slack can cause buckling and breaking of the band. Also, the slack can allow the first drum to start rotating before the second drum at the start of rewinding, which can produce a high stress in the band when the slack has been taken up. The foregoing problems can reduce the service life of the band.
With a view to overcoming these problems, an object of this invention is to provide a belt drive device which does not allow any slack in the band when it is wound around the second drum.